Wireless sensor and reader systems may be designed to wirelessly monitor the status of a remote sensor. Some such wireless systems include a sensor that transduces a physical parameter into a signal frequency. A reader is then configured to receive and measure the frequency of the sensor signal.
FIG. 1 illustrates an example of an operational frequency bandwidth of a wireless sensor/reader system and the corresponding parameter. As shown, the corresponding parameter is pressure, however it will be appreciated that the concept described herein may apply to any transduced parameter. The exemplary frequency range of the illustrated wireless sensor is from 13 to 14 MHz, which corresponds to absolute pressures of 550-900 mmHg. In the example shown in FIG. 1, frequency is inversely proportional to pressure.
In wireless sensor/reader systems, the sensor may be stimulated by a transmit pulse from a reader, causing the sensor to emit a ring back or “ring” signal at its resonant frequency once that stimulus is removed. The reader may measure the frequency of the ring signal and use a calibration table or formula to determine the sensed pressure.
The ring signal, as received at the reader, may be low power and may decay very quickly, particularly if the distance between sensor and reader is great. This is a problem with all similar wireless sensor systems, whether the systems utilizes a transmit signal that is fixed or swept. Other types of wireless sensor systems, such as those based on grid-dip techniques, may require a relatively long time and many transmit cycles to identify the sensor's resonance frequency, especially when the possible range of resonance frequencies is large.
Some wireless reader/sensor system designs require a gauge pressure reading, meaning pressure relative to local atmospheric pressure. In such designs, however, the sensor is often located at a position where it cannot access atmospheric pressure and thus cannot directly deliver a gauge pressure reading. For example, a blood pressure sensor implanted in the pulmonary artery is not capable of directly accessing atmospheric pressure. To deal with certain medical conditions, clinicians typically wish to know the gauge pressure of the pulmonary artery across a range of 100 mmHg. However, the implanted sensor has no way of knowing what the local atmospheric pressure is. In other words, the implanted sensor is only capable of sensing absolute pressure.
One solution is to place an ambient pressure sensor in the reader. The reader then measures absolute pressure from the implanted sensor, as well as absolute atmospheric ambient pressure from its ambient pressure sensor, and subtracts the ambient pressure from the absolute pressure to obtain gauge pressure.
The example in FIG. 1 illustrates a pressure range between 550-900 mmHg absolute. Ambient pressures in the inhabited regions of earth typically range from 550-800 mmHg absolute. Thus, to measure 0-100 mmHg gage, a sensor's absolute range must go from 550 mmHg (lowest ambient 550 mmHg plus lowest gauge 0 mmHg) to 900 mmHg (highest ambient 850 mmHg plus highest gauge 100 mmHg).
Therefore, there is a need to measure the frequency of a weak signal where the signal's full scale range is wide, but where only a small subset of that full range is used for any individual measurement.
Regardless of the method used to determine the sensor signal frequency, various circuits within the reader must be adapted or tuned to capture the maximum amount of energy in the sensor signal without capturing unwanted energy from sources other than the sensor, such as natural or man-made noise. For example, the reader's receiver antenna and internal filters, such as analog or digital filters, may be tuned to a passband that passes any possible frequency at which the sensor might resonate and rejects all frequencies outside that passband. However, widening the passbands of antennas and filters can cause problems, including higher attenuation, lower signal-to-noise ratios, and increased susceptibility to unwanted interfering signals.
Fixed frequency systems have difficulty overcoming these problems. Some swept frequency systems may attempt to overcome the problems by constantly re-tuning the receivers and filters to match the instantaneous frequency being transmitted. This, however, usually requires significant additional circuitry and processing.
Therefore, an improved method and apparatus are needed.